May rubber articles, principally automobile tires, but also including hoses, conveyor belts, transmission belts and the like are usually reinforced with textile fibers in cord form, or increasingly, with steel wire. In all such instances the reinforcing material must be firmly bonded to the rubber. This is so whether the fiber is natural, synthetic or metallic, or whether the rubber is natural or synthetic.
The conventional practice has been to prepare the tire cord by pre-treatment with a rubber latex containing a phenol-formaldehyde condensation product, wherein the phenol has almost always been resorcinol. Kaizerman, however, has disclosed the utility of N-(substituted oxymethyl)melamine compounds in place of the conventional resorcinol-formaldehyde in a latex dipping composition; see, for example, U.S. Pat. Nos. 3,212,955 and 3,361,617. In the case of polyester tire cord, such treatment is ordinarily preceeded by treatment of the tire cord with an isocyanate. By a mechanism not completely understood, the resin reacts with the fiber and rubber, effecting a firm reinforcing bond.
An alternative method entails compounding a vulcanizable rubber stock composition with the components of the phenol-formaldehyde condensation product. The components of the condensation product consist of a formaldehyde, or methylene, acceptor and a formaldehyde, or methylene, donor. The most commonly employed methylene acceptor is resorcinol; commonly employed methylene donors are the N-(substituted oxymethyl)melamines. The effect achieved is resin formation in situ during vulcanization of the rubber, creating a bond between the fiber and rubber irrespective of whether the fiber has been pretreated. This method is particularly useful with steel cord, where pretreatment has been largely ineffective.
The use of resorcinol has inherent disadvantages. One of the most frequently encountered is occasioned by the fact that resorcinol is not readily dispersed in rubber. In addition, resorcinol is toxic. The most commonly encountered side effect of its use is a dermatological reaction commonly referred to in the trade as "red hand". These difficulties are further compounded by the fact that there is a worldwide shortage of resorcinol.
In addition, there is an increased need in the industry for fiber reinforcing of rubber to survive high dynamic stress, such as flexing, which has brought about a continuing search for other and better methods for achieving high adhesive strength. Therefore, the present invention is based on an investigation to find a vulcanizable rubber stock composition comprising the components of a phenol-formaldehyde condensation product which avoids the use of resorcinol as the methylene acceptor, as well as providing equal or better bonding results.